• Journal of Electrical Engineering and Technology
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• v.13 no.3
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• pp.1060-1068
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• 2018

Droop control schemes have been widely employed in the control strategies for Multi-Terminal Direct Current (MTDC) system for its high reliability. Under the conventional DC voltage-active power droop control, the droop slope applies a proportional relationship between DC voltage error and active power error for power sharing. Due to the existence of DC network impedance and renewable resource fluctuation, there is inevitably a DC voltage deviation from the droop characteristic, which in turn results in inaccurate control of converter's power. To tackle this issue, a piecewise droop control with DC voltage dead band or active power dead band is implemented into controller design. Besides, an advanced droop control scheme with versatile function is proposed, which enables the converter to regulate DC voltage and AC voltage, control active and reactive power, get participated into frequency control, and feed passive network. The effectiveness of the proposed control method has been verified by simulation results.

• Journal of Electrical Engineering and Technology
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• v.10 no.6
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• pp.2249-2255
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• 2015

The use of voltage source converter multi-terminal direct current (VSC-MTDC) systems is anticipated to increase from the introduction of wind farms and super grids in the near future. Effective control of the DC voltage in VSC-MTDC systems is an important research topic. This paper proposes a new dynamic reference-based voltage droop control to control the DC voltage in VSC-MTDC systems more effectively. The main merit of the dynamic reference-based voltage droop control is that it can reduce the steady-state error in conventional voltage droop control by changing references according to the system operating conditions. The performance of the proposed control was tested in a hardware-in-the-loop simulation (HILS) system based on the OPAL-RT real-time digital simulator and four digital signal processing boards.

• The Transactions of the Korean Institute of Power Electronics
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• v.18 no.4
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• pp.387-396
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• 2013

This paper investigates the droop control of parallel inverters for an islanded mode of microgrid. Frequency and voltage droop control is one of power control and load demand sharing methods. However, although the active power is properly shared, the reactive power sharing is inaccurate with conventional method due to the unequal line impedances and the power coupling of active - reactive power. In order to solve this problem, an improved droop method with virtual inductor concept and a voltage and current controller properly designed have been considered and analyzed through the PSiM simulation. The performance of improved droop method is analyzed in not only low-voltage line but also medium voltage line.

• Journal of Electrical Engineering and Technology
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• v.13 no.2
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• pp.620-630
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• 2018

In this paper, a positioning method of distributed power system is proposed to minimize the average voltage variation of a DC microgrid through voltage sensitivity analysis. The voltage sensitivity under a droop control depends on the position of the distributed power system. In order to acquire a precise voltage sensitivity under a droop control, we analyzed the power flow by introducing a droop bus with the considerations of the droop characteristics. The results of the positioning method are verified through PSCAD/EMTDC simulation.

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.53 no.4
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• pp.255-261
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• 2004

This paper presents an improved droop method which minimizes the voltage droop of a parallel-connected power supply Conventionally, the droop method has been used to achieve a simple structure and no-interconnections among power sources. However, it has a trade-off between output voltage regulation and load sharing accuracy In this paper, the droop is minimized with a current and droop gain control using steady-stage estimation. The proposed method can achieve both good voltage regulation and good load sharing. A design example of two 10㎸, 100㎃ parallel modules is made and tested to verify the proposed current-sharing method.

• The Transactions of the Korean Institute of Power Electronics
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• v.20 no.1
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• pp.81-90
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• 2015

This paper investigates the droop control-based real and reactive power load sharing with a virtual inductor when the line impedance between inverter and Point of Common Coupling (PCC) is partly and unequally resistive in high-capacity systems. In this paper, the virtual inductor method is applied to parallel inverter systems with resistive and inductive line impedance. Reactive power sharing error has been improved by applying droop control after considering each line impedance voltage drop. However, in high capacity parallel systems with large output current, the reference output voltage, which is the output of droop controller, becomes lower than the rated value because of the high voltage drop from virtual inductance. Hence, line impedance voltage drop has been added to the droop equation so that parallel inverters operate within the range of rated output voltage. Additionally, the virtual inductor value has been selected via small signal modeling to analyze stability in transient conditions. Finally, the proposed droop method has been verified by MATLAB and PSIM simulation.

• Proceedings of the KIEE Conference
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• 2003.04a
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• pp.208-211
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• 2003

This paper presents an improved droop method of the high voltage DC power supply which minimizes the voltage droop of a parallel-connected power supply. Conventionally, the droop method has been used to achieve a simple structure and no-interconnections among the power sources. However, it has a trade-off between output voltage regulation and load sharing accuracy. In this paper, the droop is minimized with a current and droop gain control using steady-stage estimation. The proposed method can achieve both high performance voltage regulation and load sharing. Two 10kV, 100mA parallel power modules were made and tested to verify the proposed current-sharing method.

• Proceedings of the KIPE Conference
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• 2017.11a
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• pp.51-52
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• 2017

This paper presents an advanced DC droop using both an output current and a grid current. To control parallel-connected converters without communication, the DC droop control is conventionally used. The conventional DC droop control method droops output voltage using an output current. It cannot control the source current causing output voltage errors. This paper proposed the DC droop method using both an output current and a grid current to improve dynamic response of voltage droops. The simulation results with PSIM is provided.

• Journal of Power Electronics
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• v.17 no.1
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• pp.294-304
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• 2017

This paper proposes a point of common coupling (PCC) voltage compensation method for islanding microgrids using an improved power sharing control scheme among distributed generators (DGs) without communication. The PCC voltage compensation algorithm is implemented in the droop control scheme to reduce the PCC voltage deviation produced by the droop controller itself and the voltage drop on the line impedance. The control scheme of each individual DG unit is designed to use only locally measured feedback variables and an obtained line impedance to calculate the PCC voltage. Therefore, traditional voltage measurement devices installed at the PCC as well as communication between the PCC and the DGs are not required. The proposed control scheme can maintain the PCC voltage amplitude within an allowed range even to some extent assuming inaccurate line impedance parameters. In addition, it can achieve proper power sharing in islanding microgrids. Experimental results obtained under accurate and inaccurate line impedances are presented to show the performance of the proposed control scheme in islanding microgrids.

• Journal of Power Electronics
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• v.16 no.1
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• pp.329-339
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• 2016

Owing to mismatched feeder impedances in an islanded microgrid, the conventional droop control method typically results in errors in reactive power sharing among distributed generation (DG) units. In this study, an improved droop control strategy based on secondary voltage control is proposed to enhance the reactive power sharing accuracy in an islanded microgrid. In a DG local controller, an integral term is introduced into the voltage droop function, in which the voltage compensation signal from the secondary voltage control is utilized as the common reactive power reference for each DG unit. Therefore, accurate reactive power sharing can be realized without any power information exchange among DG units or between DG units and the central controller. Meanwhile, the voltage deviation in the microgrid common bus is removed. Communication in the proposed strategy is simple to implement because the information of the voltage compensation signal is broadcasted from the central controller to each DG unit. The reactive power sharing accuracy is also not sensitive to time-delay mismatch in the communication channels. Simulation and experimental results are provided to validate the effectiveness of the proposed method.